Guitarhacker: "So.... essentially, if you hadn't said a word about it, no one would have ever guessed where you got the inspiration from..... is that correct?"

I guess that would depend on the listener; some people can hear things like that! But essentially, yeah. I'm not thinking of anything you could call an "interpretation" of the original, as a normal listener would hear it. A normal listener can recognize a theme in a different key, or a different mode, but not inverted.

But regardless of normal listeners and recognizeability, it's a trivial matter to reverse-engineer a reversed piece back into the original. And I don't know how such things actually play out in courts, but I can imagine a lawyer playing a contested song on a laptop and showing how you just do "select all" then take a certain menu option and suddenly you're listening to "Raindrops Keep Fallin On My Head" – and the jury going, whoa, it's the same song.

I suppose what you do to finesse the legal situation is take your inversion of "Raindrops" and call it something like "Neutrons Keep Shooting From the Ground" where the listener will get the winky-winky meaning.

In terms of "obligations", I would just feel dishonest if I presented such as I've described as completely original material. Plus it would be too easy to get caught, once any suspicions were raised!